Polyurethane ureas are generally prepared by reacting an isocyanate group containing prepolymer with polyamines. Polyurethane ureas are of considerable technical importance and are widely used, for example, for coatings and impregnations, for the manufacture of sheets and films and in the field of elastomers.
According to the present state of the art, various methods are used for controlling the exothermic reaction of polyamines with polyisocyanates. For example, one method frequently used involves using polyamines which, by virtue of their chemical nature, are less reactive with polyisocyanates. They are mainly aromatic polyamines whose reactivity is reduced by electron-attracting substituents on the aromatic nucleus. These aromatic polyamines may be homogeneously mixed with isocyanate prepolymers in suitable machines. The mixtures remain liquid for a sufficient length of time to enable them to be cast, sprayed or applied by knife-coating.
Aromatic polyamines do, however, have various disadvantages; for example, they are, in some cases, physiologically suspect an they undergo yellowing in air and light so that they are unsuitable for the production of light-fast coatings and lacquers.
Aliphatic and cycloaliphatic diamines which do not give rise to these difficulties could not hitherto be converted to polyurethane ureas by any satisfactory process. According to the present state of the art, the preparation of polyurethane ureas from aliphatic polyamines may only be carried out in a dilute phase, for example, using a large excess of solvent. It is only in a second stage of the process that the polyurethane urea may be worked-up into the end-product, for example, a coating, and this operation is accompanied by evaporation of the solvent, which not only requires expenditure of energy, but also has the disadvantage that the solvent must be recovered for ecological and economic reasons.
A reactive coating process of the type which may be carried out using aromatic polyamines and polyisocyanates, in which the polyurea forms on the substrate after the components have been mixed, cannot be carried out using aliphatic or cycloaliphatic polyamines in the present state of the art owing to the high reactivity of these amines. Attempts have therefore been made to reduce the reactivity of aliphatic and cycloaliphatic diamines by blocking the amino groups. This means that instead of using the free diamines, the process is carried out, for example, using reaction products of the amines which aldehydes and ketones, that is to say with bisaldimines or bisketimines. Lacquers obtained on this basis have been described, for example, in German Offenlegungsschrift No. 2,325,824. (U.S. Pat. No. 3,932,357) According to the teaching in this Offenlegungsschrift, the bisaldimines used as hardeners are produced by distilling off the water formed in the reaction between a polyamine and an aldehyde.
Another method of preparing the ketimine compounds used as hardeners, which have been described in German Offenlegungsschriften No. 1,694,356 and 2,037,458 (U.S. Pat. Nos. 3,462,416, 3,463,748, 3,574,127, 3,645,907 and 3,699,063), and U.S. Pat. No. 3,715,338, consists of removing the water produced by the reaction by means of an inert drying agent, for example, a molecular sieve.
Coatings and lacquers having satisfactory properties may be obtained from such blocked diamines but the process is technically not completely satisfactory, for the following reasons:
Diamines must again be formed from the bisaldimines and bisketimines before the polymeric polyurethane urea may be formed. This requires water which is absorbed from the surroundings of the film, for example, from the moisture in the atmosphere. This means that the properties of such lacquers and coatings depend to a large extent on the moisture content of the surrounding air. Since this moisture content is, of course, liable to vary widely, difficulties in reproducing the results often arise. Moreover, the moisture enters the film from above so that only the uppermost layer becomes hardened sufficiently rapidly while the lower layers may, in some cases, take days to harden completely. In the case of relatively thick films, this has the result that the polyurethane urea obtained is non-homogeneous and the materials attain their final strength only after some time. For this reason, it is also impossible to produce polyurethane urea coatings by a reactive process from such bisketimine hardeners and isocyanate prepolymers using modern coating machines. When the two reactive components have been mixed and sprayed on the substrate, the coating runs through a drying channel and by the time it reaches the end of this channel, it must be completely hardened and in a suitable condition for stacking. However, since the time available in the machine is at most about 6 minutes, the coatings do not harden sufficiently under the conventional operating conditions.
It has now been found that these difficulties may be overcome and polyurethane ureas which harden rapidly may be obtained by a low solvent process if isocyanate prepolymers in a low solvent phase are mixed with water and with aliphatic, cycloaliphatic or araliphatic diamines in which the NH.sub.2 groups are partially blocked. It is surprisingly found that when this method is employed, the rate of hardening of the films and lacquers may virtually be controlled as desired depending upon the quantity of water used and may be adapted to the given conditions. The most surprising and in fact completely unexpected result is that, in spite of the excess of water present, practically no reaction takes place between the water and the isocyanate and the coatings are in no way foamed or flawed with bubbles due to evolution of CO.sub.2.